Understanding Cardiac Thick Filament Regulation through X-ray Diffraction: Recent Advances and Future Directions

The contraction of the heart muscle is achieved by cyclic interactions between actin and myosin filaments within the sarcomere, the fundamental contractile unit of muscle. Although decades of biochemical and structural studies have identified the molecular components responsible for force generation, many of the structural transitions that regulate contraction occur within the sarcomeres and remain inaccessible to conventional structural techniques. The highly ordered paracrystalline arrangement of myofilaments in striated muscle makes them uniquely amenable to synchrotron small-angle X-ray diffraction, which enables direct, non-destructive measurements of myofilament structure under near-physiological conditions and on physiologically relevant timescales, As the only technique capable of monitoring these structural changes in functioning muscle, X-ray diffraction has provided otherwise unobtainable insights into the molecular mechanisms of cardiac contraction. Recent discoveries of thick filament-based regulation, together with advancement of experimental methodologies have transformed cardiac X-ray diffraction into a powerful tool for uncovering the structural basis of cardiac physiology, cardiomyopathies, and the mechanisms of action of emerging sarcomere-targeted therapeutics. In this review, I highlight these recent developments and discuss how they are reshaping our understanding of cardiac muscle regulation in health and disease.